Functional dyes are compounds having various characteristics such as a light-emitting property, photoconductivity, light absorptivity, an energy conversion property, and the like. It is expected that, through the use of such physical properties and reactivity, the functional dyes be applied to cutting-edge fields such as electronics, photonics, and molecular imaging, in particular, information recording that responds to external light, heat, pressure, electric field, and the like, information display, energy conversion, medical diagnosing, agricultural and gardening fields, and the like.
Rhodamine dyes, which are one of the most famous functional dyes, are pi-electron organic dyes having characteristics of: (1) emitting fluorescence in a long-wavelength region; (2) having a high fluorescence quantum yield; (3) having a high water-solubility; and (4) having a high resistance to photobleaching. The rhodamine dyes are widely used as fluorescent probes in molecular biology, and widely used in dye-sensitized solar cells, dye lasers and the like, through the use of these characteristics.
However, the rhodamine dyes tend to form aggregates when the rhodamine dye are dissolved in a solution in high concentration or doped on solids. When forming aggregates, the rhodamine dyes form dimers and cause a π-π stacking. At this time, hydrophobic sites of luminophores in molecules overlap each other face-to-face and form non-emission H-type aggregates. As such, the rhodamine dyes have a problem that due to aggregation under the high-concentration condition, luminous efficiency, color development, photosensitivity, and photosensitization are decreased. Note that, the decrease in the functionality due to the aggregation is a phenomenon that conventional organic functional dyes exhibit, and occurs regardless of whether the functional dyes are rhodamine dyes or not. This phenomenon is the largest factor that limits an applicable range of the functional dyes.
In view of the above problems, Tang and others have reported that a Silole compound exhibits AIEE, i.e., emits light when forming aggregates (Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun., 4332-4353 (2009)). According to the report of Tang and others, various derivatives are produced. However, these compounds still exhibit low fluorescence quantum yield. That is, no pi-electron AIEE molecule having a high luminous efficiency has been developed so far.